1. Field of the Invention
This invention relates generally to radio navigation systems of the Doppler type, and particularly to remote angle determining receivers therefor.
2. Description of the Prior Art
In Doppler radio navigation systems, the ground beacon (commutated array) transmits bidirectional scans defined as involving:
A. COMMUTATION OF A FIRST RADIO FREQUENCY DISCRETELY AND SUCCESSIVE TO THE RADIATING ELEMENTS OF A LINEAR ARRAY SO AS TO SIMULATE CONSTANT VELOCITY BIDIRECTIONAL (BACK AND FORTH) SCANNING OF THE ARRAY, WHEREBY A MAIN BEARING SIGNAL IS TRANSMITTED WHICH, AS "SEEN" BY A REMOTE RECEIVER IN THE SYSTEM, UNDERGOES A Doppler frequency shift proportional to the sine of the angle said remote receiver makes with the normal to the axis of the array.
B. Simultaneous transmission of a reference signal of a second radio frequency (offset from the first), whereby the receiver is able to derive a Doppler beat waveform containing a frequency indicative of the said angle.
C. Changing the sign of the difference (offset) between the commutated frequency and the reference frequency at each half period of the bidirectional scan, thereby to provide compensation for the change in the sense of the Doppler frequency shift imparted to the commutated signal on reversal of the direction of scan.
D. PRECESSION OF THE PHASE OF THE COMMUTATED SIGNAL AT THE COMMENCEMENT OF EACH BIDIRECTIONAL SCAN, WHEREBY SUCCESSIVE SCANS ARE NOT IDENTICAL (A FORM OF DIGITIZATION).
A form of beacon having the above transmission format is described in British Pat. No. 1,234,541. The specification of that patent also describes how the Doppler frequency displacement of the commutated signal may be detected and measured by digital methods in an associated remote receiver.
In order to effect digital counting of the Doppler frequency in the receiver in such a way that the signal is measured only during defined portions of the scan, it is necessary to derive scan timing signals to control the time over which the period of received signals are measured by a computing counter. This can be done by examining the r.f. signal and detecting when the frequency of the commutated signal changes at the end of each scan. However, the circuitry required for this detection is quite complicated.